Method for recognising digital fingerprints by distortion and computer system for using said method

ABSTRACT

The invention relates to a digital fingerprint recognition method comprising the following steps consisting in: activating an image distortion software program on a computer ( 3 ); displaying a digital fingerprint image on a screen ( 1 ) of the computer ( 3 ) in a presentation environment for said image distortion software; and distorting the digital fingerprint image so as to correct a fault in the fingerprint detected on said digital fingerprint image. The computer system comprises a man/machine interface ( 1, 2 ), a processing unit ( 4 ) that is used to analyse a digital fingerprint image, means ( 9 ) for accessing one or more files containing at least one digital fingerprint image and an image distortion software program that is used to display a digital fingerprint image on the man/machine interface ( 1 ) and to distort the digital fingerprint image by means of the man/machine interface ( 1, 2 ).

[0001] The field of the invention is that of the recognition offingerprints.

[0002] The recognition of fingerprints is commonly used to identity aperson through his fingerprints.

[0003] A database of fingerprints generally catalogs a multitude offingerprint images with a set of characteristic points for each one anda match with an identity of someone to whom the fingerprint isattributed.

[0004] A fingerprint image consists of a certain number of relativelydark lines separated by relatively light lines with stopping points andpoints of modification of curvature that constitute the characteristicpoints of the fingerprint.

[0005] To establish a one-to-one match between an image that resultsfrom the taking of a print of unknown identity and an image of a printfrom the database, one proceeds overall in the following manner. Theunknown print image is presented to a specialist who determinescharacteristic points of the unknown print. The unknown print imagetogether with its determined characteristic points, is submitted to acomputer system which compares it with the images of the database. Thecomputer system picks out from the images of the database, a sample ofthose that an algorithm judges in a known manner, to be the closest tothe image submitted. The images picked out by the computer system arethereafter presented to the specialist who compares them with theunknown print image so as to find a match between the unknown printimage and a print image from the database.

[0006] A problem arises when the unknown identity of the print resultsfrom having taken it from some arbitrary surface, in particular anon-planar surface. This is the case, for example, in a police inquiryduring which fingerprints are taken from any type of object. The resultsare not always as accurate as for fingerprints that result from the evenpressure of a finger on a planar surface specifically provided for thispurpose. In particular, certain lines may be distorted, on account forexample, of local deformation of the skin due to its elasticity or of adistortion of the supporting object while the print is being deposited.

[0007] To determine the characteristic points before submission to thecomputer system, the specialist is confronted with an effort ofimagination that is continuously sustained during the observation of theimage to interpret whether, for example, a break in line curvature givesrise to a characteristic point determination or whether it seems ratherto result from a fault with the taking of the print by comparison withthe whole image.

[0008] When several fingerprint images, picked out by the computersystem as having characteristic points of the fingerprint of unknownidentity, are presented to the specialist so that he may choose the onehe judges to be closest to the image under study, the specialist isagain confronted with an effort of imagination in order to compare eachimage presented with the image submitted. On account of absence ofsimilarity that results from faults with the taking of prints, thespecialist must raise the matter of ascertaining for each imagepresented whether an absence of similarity results from the fact thatthe fingerprint is different or from the fact that the print taken isspoilt. This additional intellectual effort impedes efficiency andinduces fatigue in the specialist.

[0009] To alleviate the aforementioned drawbacks, a first subject of theinvention is a method of fingerprint recognition comprising stepsconsisting in:

[0010] activating an image distortion software program on a computer;

[0011] displaying a fingerprint image on a screen of the computer in apresentation environment of said image distortion software program;

[0012] distorting the fingerprint image so as to correct a faultdetected on said fingerprint image.

[0013] The fault is for example a fault in taking the print, a localdeformation of the print due to uneven pressure of the finger on thesurface from which the print is taken owing to the elasticity of theskin or the nature of the support.

[0014] By using this method, a specialist can fix on the fingerprintimage, the result of his analysis of each fault with the taking of thefingerprint in order to eliminate the faults that he detects from theimage. The specialist can then analyze the characteristic points of thefingerprint image without needing continuous effort to take mentalaccount of the faults of the image.

[0015] A second subject of the invention is a computer system comprisinga man/machine interface and a processing unit, characterized in that itcomprises:

[0016] means of access to one or more files containing at least onefingerprint image;

[0017] an image distortion software program for displaying a fingerprintimage on the man/machine interface and for distorting the fingerprintimage by means of the man/machine interface.

[0018] By means of this computer system, the specialist can use themethod which is the subject of the invention.

[0019] Further details and advantages of the invention will emerge froman example of use, described with reference to the appended drawings inwhich:

[0020]FIG. 1 diagrammatically shows a computer system in accordance withthe invention;

[0021]FIG. 2 shows method steps in accordance with the invention;

[0022]FIG. 3 shows a deformed image of a fingerprint;

[0023]FIG. 4 shows a corrected image of a fingerprint obtained afterdistortion of the image of FIG. 3.

[0024] With reference to FIG. 1, a computer system comprises aman/machine interface with a screen 1 and a mouse 2. A computer 3comprises a processing unit 4 such as a microprocessor, a memory 5 and abus 7 to which the processing unit 4 and the memory 5 are linked in aknown manner. A disk 8 comprises one or more files containing at leastone fingerprint image digitally coded point by point, for example in the*.bmp format known from office computing.

[0025] The memory 5 contains an image distortion software programexecutable by the processing unit 4. A coupler 9, linked on the one handto the disk 8 and on the other hand to the bus 7, allows the processingunit 4 to access the image files of the disk 8 so as to load at leastone thereof into memory 5. A coupler 6, linked on the one hand to thescreen 1 and to the mouse 2 and on the other hand to the bus 7, allowsthe processing unit 4 to display on the screen 1 a fingerprint image inan environment of the image distortion software program.

[0026] The image distortion software program contained in the memory 5is devised so as to dispose by means of the mouse 2, anchoring points10, 11, 12, 13, 14, 15 on the fingerprint image displayed on the screen1. The software catalogs in memory the planar coordinates of eachanchoring point disposed by means of the mouse.

[0027] Moreover, for each pair of planar coordinates defining an imagepoint, the image distortion software program utilizes values ofattributes such as levels on a grayscale so as to represent the darklines and the light lines of the fingerprint on the screen 1.

[0028] The image distortion software program is devised so as to move ananchoring point 12 by means of the mouse 2, while leaving the otheranchoring, points 10, 11, 13, 14, 15 fixed and to do this operation bypermuting any anchoring point disposed on the fingerprint image.

[0029] The image distortion software program is devised to recalculatethe coordinates of each image point subsequent to a movement of ananchoring point, so as to model a planar plastic distortion of theimage.

[0030] The modeling is for example programmed in such a way that thecoordinates of a point labeled by its attribute values, are recalculatedso as to comply with the mechanical rules of distortion of media. Thatis to say each point of the image is moved in a manner analogous to apoint of a plastic surface subjected to compressive and elongationalloads with respect to the anchoring points that have remained fixedaccording to the initial position that this point of the image occupieswith respect to the moved anchoring point and according to the sense andthe direction of movement of the moved anchoring point. Thus,considering for example, a movement of a first anchoring point thatshifts the first anchoring point closer to a second, anchoring pointthat has remained fixed while shifting the first anchoring point furtheraway from a third anchoring point that has remained fixed, it ispossible to observe a reduction in spacing between the lines situatedbetween the first anchoring point and the second anchoring point and anincrease in spacing between the lines situated between the firstanchoring point and the third anchoring point.

[0031] With reference to FIG. 2, a fingerprint recognition method inaccordance with the invention comprises a step 16 in which the imagedistortion software program is activated so as to display on the screen1 a usage environment of the image distortion software program.

[0032] In a step 17, a fingerprint image is displayed on the screen 1,within the environment of the image distortion software program.

[0033] In a step 18, the fingerprint image is observed so as to detectany faults with the taking of the print as may be done by a specialistwith his own innate know-how.

[0034] If no fault is detected, the characteristic points of thefingerprint image are analyzed in a step 21 so as to continue acustomary method of fingerprint image recognition.

[0035] If a fault is detected, anchoring points are placed on and or inthe neighborhood of the fault in a step 19. To do this, the cursor ofthe mouse is brought to the screen over the point of the image where onewishes to place an anchoring point and one clicks, for example, on theleft button of the mouse. After having placed the last anchoring point,one clicks for example on the right button of the house.

[0036] In a step 20, one or more anchoring points are moved in such away as to correct the fault detected. To do this, the cursor of themouse is brought onto the anchoring point to be moved. One presses forexample on the left button of the mouse and without releasing thepressure, one moves the cursor to the new desired position of theanchoring point. One then releases the button of the mouse, therebycausing a recalculation of the coordinates of the points of the image bythe image distortion software program.

[0037] Steps 18 to 20 are repeated until there is no longer anydetection of fault in step 18.

[0038]FIG. 3 shows a deformed fingerprint image on which a specialisthas placed four anchoring points 22, 23, 24, 25 since he has detected afault in the neighborhood of the anchoring points 23 and 24.

[0039]FIG. 4 shows the fingerprint image after the specialist has movedthe anchoring point 23 in the sense and the direction of the arrow 26presented in FIG. 3 and the anchoring point 24 in the sense and thedirection of the arrow 27, the anchoring points 22 and 25 remainingfixed. The fault detected in the image of FIG. 3 has disappeared in theimage of FIG. 4.

[0040] It is observed that the lines of the image of the fingerprinthave hardly varied outside the neighborhood of the fault and that thecharacteristic points are preserved so as to allow analysis thereof. Thespecialist can concentrate his attention on the corrected image of FIG.4 without having to worry about the fault which has then disappeared.

[0041] Moreover, the coordinates of the characteristic points in theplane of the image, which are then determined with greater accuracy,facilitate the comparison with characteristic points on knownfingerprint images cataloged in databases.

1. A method of fingerprint recognition characterized in that itcomprises steps consisting in: activating an image distortion softwareprogram on a computer (3); displaying a fingerprint image on a screen(1) of the computer (3) in a presentation environment of said imagedistortion software program; distorting the fingerprint image so as tocorrect a fault detected-on said fingerprint image.
 2. The method offingerprint recognition as claimed in claim 1, characterized in that inthe step consisting in distorting the fingerprint image, anchoringpoints are disposed in a neighborhood of the fault such as it isdetected and at least one anchoring point is moved so as to correct thedetected fault.
 3. A computer system comprising a man/machine interface(1, 2) and a processing unit (4) for analyzing a fingerprint image,characterized in that it comprises: means of access (9) to one or morefiles containing at least one fingerprint image; an image distortionsoftware program for displaying a fingerprint image on the man/machineinterface (1) and for distorting the fingerprint image by means of theman/machine interface (1, 2).
 4. The computer system as claimed in claim3, characterized in that the image distortion software program isdevised so as to dispose at least two anchoring points on the imagedisplayed and so as to move one of the anchoring points, the otherremaining fixed, in such a way as to be able to subject the image to aplastic distortion by moving the said one anchoring point with respectto the other said anchoring point that has remained fixed.